The invention relates to a circuit and method for monitoring the operational reliability of rechargeable lithium cells, by measuring and monitoring the cell voltage and the cell temperature.
Rechargeable lithium/ion cells require a special charging technique and, in particular, predetermined switch-off voltages must be complied with exactly while, if the charger fails, a protective circuit must prevent and inhibit further charging. When such cells are being discharged, specific voltage lower limits must not be infringed, to avoid irreversible damage to the cell. Lithium-ion cells are thus provided with special electronic protective switches which switch off at predetermined final charging and discharge voltages if the charger or the appliance behaves incorrectly and, together with special protection devices, are intended to protect the cell against unacceptable voltage levels and currents, and to prevent dangerous situations occurring, which could result in irreversible decomposition of the electrolyte and cell damage. Chargers with such monitoring functions are described, for example, in WO96/15563.
Thus, it would be advantageous to provide a method which further improves the operational reliability of rechargeable lithium cells.
This invention relates to a method for monitoring operational reliability of rechargeable lithium cells including measuring cell voltage and cell temperature, and discharging the cell if a predetermined cell limit voltage (UG) and a predetermined ambient limit temperature (TG) are exceeded substantially at the same time, until a predetermined lower voltage level or a predetermined lower ambient temperature is reached.
This invention also relates to a circuit arrangement for carrying out the above method and includes a first threshold value switch (D) connected to respond when a predetermined upper limit voltage (UG) is exceeded in the cell, a second threshold value switch (C) connected to respond when a predetermined ambient temperature limit (TG) is exceeded, and a logic circuit (E) which connects load resistance (G) to a load when the first and second threshold value switches (D) and (C) respond at substantially the same time, and wherein the first and second threshold value switches (C) and (D) are configured such that they trip when a lower cell voltage or a lower ambient temperature is reached.